In a gaseous reservoir the pressure may drop relatively fast, which results in diminishing production. After some time of production, the pressure in the gaseous reservoir may have dropped to a pressure that is lower than the pressure from the water at the seabed/sea bottom/water bottom above the reservoir. Thus it becomes feasible to inject water in the lower water containing parts of the reservoir without any injection pump or other pressure support, but only by arranging an injection well in which water can flow through due to high hydrostatic pressure at the seabed at the inlet of the injection well.
In order to support the production of gas flow from a gas reservoir, it is known to arrange a gas compressor at the outlet of the production well, either at the seabed or at the surface installation. Gas compressors are usually operated/driven electrically. For gaseous reservoirs with condensate/oil, multiple face machines or -pumps may alternatively be installed, which are typically electrically operated.
In the patent publication RU2109930, a method for developing gas reserves below the continental shelf is disclosed. The method aims to avoid use of the pressure increasing compressor station with compression of gas prior to further transport. Further the aim is to reduce the costs for developing the reservoir to a commercial level. At a predetermined moment the injection wells are put into operation in order to maintain the pressure with pumpless injection of seawater in the reservoir wells by utilizing the pressure of the water column between the sea surface and the well head level. Water injection without pumping utilizes the pressure head in the water column between the sea surface and the level of the well head at the seabed. The advantage according to the teaching in the mentioned publication is development of the field with reduced capital costs due to delivery of gas to the gas pipes without use of compressors, and maintaining the reservoir pressure without pumping. Possibility for installation of hydraulic turbines in the injection wells for production of electricity are mentioned.
In the patent publication WO 02/33218 A1 methods and arrangements for treatment of fluid are disclosed. Furthermore, methods and arrangements are disclosed to utilize the energy in water flowing from a high pressure reservoir. For gas production and gas/condensate/oil production from fields with low and diminishing pressure, there is a need for methods and assemblies which may deliver energy to the gas flow or the gas/condensate/oil flow to maintain the production as well as contribute to maintain the pressure in the reservoir during diminishing reservoir pressure. The objects of the present invention are to meet the above mentioned requirements.